The most widely used stylus type electrostatic recorders today apply a charge to a recording medium through a pair of coincident voltage pulses applied to opposite sides of the recording medium by means of a plurality of stylus electrodes and backup or counter electrodes. The backup electrode arrangement usually takes the form of a plurality of U-shaped shoes fabricated from a metal block having an elongated channel. The U-shaped backup electrode arrangement, as compared, for example, to the employment of a flat plate backup electrode arrangement, provides reduced wear on the stylus electrodes and its head assembly while also protecting to some extent the stylus electrodes from dielectric breakdown.
Head wear in this older arrangement was reduced because the spaced, but parallel, lips of the U-shaped backup electrode provided minimal contact with the recording medium, permitting the medium to freely adjust to accomodate for small physical fluctuations in the profile of the stylus head. Complete dielectric breakdown was minimized because the effective resistance of the recording medium in contact between the backup electrode lips and the stylus electrodes provided sufficient protection against pinhole shorts in the recording medium which would otherwise cause machine damage to the stylus electrodes and recorder circuitry.
Because there is an effective recording medium increment included in the charging path between the stylus electrode array and the lips of the U-shaped backup electrodes, parallel charging paths are created which increase the charging path impedance thereby necessitating longer write times. Longer write times (longer charge depositing time due principally to a higher RC time constant in the charging path) means a slower printing process.
One manner of improving printer speed is to decrease this impedance in the charging path while not subjecting the recorder to an increased likelihood of pinhole shorting through the recording medium while further providing efficient recording medium alignment for faster movement of the recording medium through the recording station and the development station.